Pressure polished extruded polycarbonate sheet

ABSTRACT

Pressure polished transparent unfilled polycarbonate sheet substantially free of ripples, wiggles, extrusion die lines, and stresses and having a light transmittance greater than, a percentage haze lower than and a distortion lower than the as extruded sheet so that the polished sheet can be mechanically or thermoformed into such articles as airplane windshields, canopies, protective headgears and see-through enclosures. The sheet is made by polishing it between mirror finish plates using a sequence of preheating the as extruded sheet to the heat distortion temperature at zero or minimal pressure, heating to higher temperatures while rapidly increasing the pressure so as to transfer the mirror finish from the plates to the sheet surfaces, and cooling the sheet while decreasing the pressure to fix the mirror finish in the sheet surfaces as the sheet contracts and solidifies.

This invention relates to optical grade unfilled transparentpolycarbonate sheet made by pressure polishing the extruded sheet toproduce the optical grade material and is a continuation-in-part of mycopending application Ser. No. 232,725 filed Mar. 8, 1972, now U.S. Pat.No. 3,855,042 which is in turn a division of my application Ser. No.54,185 filed July 13, 1970, now U.S. Pat. No. 3,681,483.

Polycarbonate resins are polyesters of carbonic acid and bisphenol A,are transparent and are well known for their high mechanical strengthproperties such as toughness, high impact strength and high tensilestrength over a broad temperature range and non-shatteringcharacteristics. They are extruded as sheets and as such contain ripplesand extrusion die lines or stresses which cause undesirable opticalproperties such as relatively low luminous transmittance and a highpercentage of haze. Polycarbonates do not lend themselves to removal ofthese ripples, blemishes, die lines, scratches or nicks by conventionalgrinding, buffing and polishing techniques.

It is the primary object of this invention to provide an optical gradetransparent unfilled polycarbonate sheet having luminous transmittancebetter than, haze less than and distortion less than the as extrudedsheet. Such material, being tougher than acrylics, vinyls, cellulosesand other clear thermoplastics, and having desirable optical properties,can advantageously be used to fabricate, mechanically or bythermoforming, airplane windshields, canopies, protective headgear andsee-through enclosures requiring minimal wall thicknesses and the like.

Another object of the invention is to provide polycarbonate sheets withoptical properties superior to the as extruded sheets made by anefficient, accurate and reproducible method using controlled stages ofheat and pressure and mirror polished plates to eliminate substantiallyall of the ripples, wiggles and extrusion die lines and stresses, whichproduce the inferior optical properties of the as extruded sheets.

Another object of the invention is to provide an optical gradetransparent unfilled polycarbonate sheet made by a method employing asequence of first charging a press with the as extruded sheet which isretained between plates of high mirror polish, then pre-heating thesheet slowly to its heat distortion temperature with little or nopressure applied so as to expel air entrapped between the sheet and themirror polished plates, then rapidly raising the pressure to about300-500 psi while the temperature rises to the fusion temperature atwhich time mirror surfaces are formed in the sheet and the ripples,wiggles and extrusion die lines and stresses are removed, finallycooling the material slowly to about 150°F while cutting the pressure inhalf to retain the mirror polish on its surfaces while the materialcools and solidifies. If the polycarbonate will eventually bethermoformed into its final article, to eliminate molecular memory inwhich the surfaces of the plastic sheet may revert to their originalform including the ripples, wiggles and extrusion die lines or stresses,the step of raising the temperature somewhat higher than the fusiontemperature while reducing the pressure to prevent thinning isinterposed between the fusion stage and the cooling stage.

These and other objects of the invention will become more apparent asthe following description proceeds in conjunction with the accompanyingdrawings, wherein:

FIG. 1 is a diagrammatic view of a multi-platen press shown open andcharged with the members required to initiate pressure polishing of theas extruded polycarbonate sheet;

FIG. 2 is a view similar to FIG. 1 showing the press closed;

FIG. 3 is a sectional view through the unit to be charged into thepress;

FIG. 4 is a perspective view of the final highly polished plastic sheet;

FIG. 5 is a sectional view of a modified unit to be charged into thepress; and

FIG. 6 is a flow diagram of the sequential process steps.

Specific reference is now made to the drawing in which similar referencecharacters are used for corresponding elements throughout.

Polycarbonate resins which are polyesters of carbonic acid and bisphenolA are available in various grades commercially under such trademarks asLexan (General Electric), Merlon (Mobay Chemical), etc. They have hightensile and impact strength over broad temperature range, are virtuallyshatter-proof and are extrudable as transparent sheets. Since the sheetscontain ripples, wiggles and extrusion die lines or stresses they do notpossess desirable optical properties. Moreover, they cannot be readilyground, buffed and polished by conventional means to remove theseripples, die lines and other flaws.

The steam heated press 10 employed to carry out the process isconventional and generally comprises a head 12 having an upper platen 14at its lower surface, a vertically movable bed 16 carrying a lowerplaten 18 and a plurality of vertically movable platens 20 (only oneshown as illustrative) between the upper and lower platens definingbetween them the charging and polishing position. The platens includeinterior coils which are connected by suitable couplings and flexiblehoses 22 to a source S of steam under pressure via a valve V₁, therebeing a pressure gauge 24 in the line. The platens are also connected bysuitable couplings and flexible hoses 26 to a source of cold water CWvia an appropriate valve V₂.

The press is hydraulically operated from below by means of a ram 28secured to the bed 16 and vertically movable in a cylinder 30.Appropriate conduits 32 connect the cylinder to a source of hydraulicfluid R via a pump and a pressure gauge 34. The means to raise and lowera ram and thus close and open the press are well known and can involve areversible pump P or a pair of pumps and separate hydraulic circuitry orequivalent means.

When the ram is retracted and the press is open as shown in FIG. 1, unit36 of FIG. 3 is charged therein by placing each unit on a platen spacedfrom the adjacent upper platen. The unit 36 comprises an extruded sheet38 of transparent unfilled polycarbonate, a pair of upper and lowermirror polished plates 40 coextensive with and in face to face contactwith the opposite surfaces of the plastic, a pair of upper and loweraluminum backer plates 42 in face to face contact with the mirrorpolished plates and a pair of upper and lower cardboard sheets 44 inface to face contact with the aluminum plates. When the unit ispositioned in the press the lower cardboard object contacts one surfaceof each platen and when the press is closed, as shown in FIG. 2, theupper cardboard sheet of that unit 36 contacts the undersurface of thenext upper platen.

The plate 40 is preferably a hard rolled brass press plate having apressure resistance of about 64,000 psi, offering excellent resistanceto deformation, made from a double spring hard special copper alloy witha clean alpha structure, i.e., only one kind of crystal, and istherefore capable of taking a mirror-like polish (National ElectricalAssociation of the USA designation No. 8) without any visible polishinglines. To protect the mirror polished surface, the plate may be nickelplated with a layer of about 0.00032 inch or plated on the nickel with alayer of about 0.00008 inch chromium. Such a plate is commerciallyavailable as Wieland highly mirror polished press plates sold by FabriteMetals Corporation, New York, New York. In place of the chrome mirrorplate, a heat or chemically tempered glass with highly polished parallelsurfaces can be used, or any other equivalent mirror finish plate,provided the same can withstand the pressures and temperatures employedin the process and any torque to which the plate may be subjected due touneven closure of the platens.

The aluminum backer plates 42 are employed to protect the mirrorpolished plates 40 as well as to distribute the heat evenly over theplastic sheet 38 and to reduce imperfections which may exist in theflatness of the platens.

The cardboard sheets 44 are somewhat resilient and are used as goodinsulators to even out the heating and cooling of the plastic sheet 38and to help level out high and low spots and thus prevent blisters.

Although the unit 36 is shown to be of lesser width than the platens,this is merely for illustrative purposes. The unit can desirably becoextensive with the platen.

For the pressure polishing of polycarbonate sheets up to 3/8 inch, thereis not sufficient lateral flow under the conditions of pressure andtemperature used in the process, as will appear hereinafter, to requirethat the plastic be laterally confined and hence the unit 36 will becharged into the press as such. However, such lateral flow may beencountered when the thickness of the sheet to be polished exceeds 3/8inch, in which case lateral confinement is required and for this unit 46shown in FIG. 5 is used. Thus the lower aluminum plate 48 is the baseplate of a mold upon which an aluminum frame 50 is secured to provide amold cavity 52 which is narrowed by the insertion of a frame 54 ofcompressible wood, such as pine, which extends above the aluminum frame50. The lower mirror polished plate 56 is laid in the cavity against thebase plate 48 and within the confines of the wooden frame 54 and thepolycarbonate or plastic sheet 38 is placed on the lower mirror polishedplate 56 also within the confines of the wooden frame. The thickness ofthe plastic sheet is such that its upper surface is beneath the upperedge of the wooden frame upon which is placed the upper mirror polishedplate 58. Over the upper mirror polished plate is placed the upperaluminum backer plate 60. It is to be understood that when the unit 46is charged into the press, upper and lower cardboard sheets 44 will beplaced over the upper aluminum plate 60 and below the aluminum baseplate 48 respectively.

The units 36 are charged in the press, and while only two such units areillustrated, it will be understood that many more can be employed forthe simultaneous pressure polishing without impairing the efficiency andaccuracy of the process. The press is then closed by activation of thepump P so that the ram 28 raises the bed 16 and all the platens save theupper 14, in which closure action the upper cardboard sheets engage theundersurface of adjacent upper platens in each operative station, asseen in FIG. 2. Steam is admitted to the platens via the valve V₁ andthe temperature is raised slowly while the pressure is maintained atzero (weight of the platens) up to 50 psi on the plastic sheet 38 as amaximum. The temperature to which the plastic sheet is raised is itsheat distortion temperature which, by definition, is the temperature ofthe material when it begins to distort under a pressure of 66 psi. Inthe case of polycarbonate this temperature is about 270°F and the periodover which such temperature is attained is about 30 minutes for 1/2 to3/4 inch material. In this pre-heat stage or cycle entrapped air isexpelled between the mirror polished plates 40 and the aluminum plates42 and causes even face to face contact of the mirror polished platesand the mating surfaces of the plastic sheet.

Where the unit 46 is used to laterally confine the plastic sheet at itsheat distortion temperature, the closing of the press causes the woodenframe 54 to compress and the upper mirror polished plate 58 to contactthe upper surface of the plastic sheet. The upper mirror polished plateis placed over the frame 54 and not directly on the plastic sheet forthe reason that should splinters of wooden frame form during thecompression thereof, they will embed in the edges of the plastic sheetwithout impairing the formed mirror surface and need merely be trimmed;whereas if the splinters lodged between the upper mirror polished plate58 and the aluminum backer plate 60, the plate 58 would be dented andultimately the mirror polish formed on the upper surface of the plasticsheet 38 would be marred.

In the next or heating stage, the pressure is raised rapidly to about300 to 500 psi while the temperature is allowed to increase to about310°-330°F. This temperature rise is obtained over a period of 10 to 20minutes while the pressure is maintained at said value of 300 to 500psi, depending upon the thickness of the plastic sheet. In this heatingstage or cycle the mirror polish of the plates 40 is transferred to theplastic sheet while the ripples, wiggles, and extrusion die lines andstresses are substantially eliminated.

If the polished transparent unfilled polycarbonate is destined forthermo-forming into final articles, then a second heating stage or cycleis used to destroy the molecular memory of the material so that theoriginal ripples, wiggles, and extrusion die lines and stresses will notreturn upon fabrication. In this molecular memory-destroying cycle thematerial is raised to a temperature of about 350°-360°F while thepressure is slowly reduced to about half its original value as amaximum, and preferably 50 to 75 psi, over a period of about 20 minutes.This decreased pressure prevents excessive flow and thinning of theplastic sheet.

Thereafter the material is cooled slowly to about 150°F whilemaintaining the decreased pressure at a maximum of 150 psi andpreferably 50 to 75 psi. The cooling stage is effected by shutting valveV₁ and opening valve V₂ to the cold water supply CW while maintainingthe desired ram pressure. The cooling cycle is about 45 minutes. In thefirst several minutes thereof this low pressure is maintained, that isuntil the temperature drops about 10°F. Thereafter, the pressure on thematerial can be allowed to rise while the ram pressure control is turnedoff so that as the plastic sheet cools and contracts, the press pressurewill drop accordingly.

If the polished polycarbonate will not be destined for fabrication intoa final article by thermoforming, then the second heating stage or cyclecan be eliminated and the material which was subjected to a temperatureof about 310°-330°F and a pressure of 300 to 500 psi is subjecteddirectly to the cooling cycle.

After the press is opened and the polished plastic sheet is removed, itis allowed to cool slowly in the atmosphere. Because the surfaces of thepressure polished polycarbonate sheets are substantially free ofripples, wiggles and extrusion die lines or stresses, the polishedsheets have greater luminous transmittance, lower haze and lesseroptical distortion than the as extruded sheets and thus such pressurepolished sheets can be used advantageously to fabricate such articles asairplane windshields, canopies, protective headgear and other enclosuresrequiring good see-through properties but minimal wall thickness.

The polycarbonate sheets of various thicknesses which were pressurepolished by the herein-described process were compared to the samethicknesses of unpolished as extruded polycarbonate sheet as to percentluminous transmittance and percent haze using the ASTM D1003 Procedure Bwith the following results, it being understood that when a particularthickness of sheet is sold commercially there is a ±10% variationthereof included, e.g. 0.040 inch polycarbonate sheet will commerciallycover from 0.036 to 0.044 inch in thickness:

                  TABLE                                                           ______________________________________                                        Thickness          % Total Luminous                                           of                 Transmittance                                                                              % Haze                                        Polycarb.                                                                             Sold                                                                  Sheet   Commercially                                                                             As       Pol-  As     Pol-                                 (Inches)                                                                              As         Extruded ished Extruded                                                                             ished                                ______________________________________                                        0.038              93.0           3.23                                        0.038              87.2           4.01                                                0.040"                                                                0.040                       87.5         2.74                                 0.040                       95.4         1.68                                 0.128              87.4           2.40                                        0.124              88.2           2.38                                                1/8"                                                                  0.129                       86.5         1.62                                 0.129                       90.0         2.67                                 0.246              79.9           6.01                                        0.246              79.9           6.01                                                1/4"                                                                  0.248                       85.1         2.35                                 0.248                       85.7         2.33                                 0.380              84.2           3.09                                        0.379              83.5           3.11                                                3/8"                                                                  0.370                       83.0         2.53                                 0.370                       80.7         2.73                                 0.498              77.0           2.73                                        0.498              75.6           3.17                                                1/2"                                                                  0.491                       79.0         3.04                                 0.486                       82.0         2.68                                 0.735                       74.0         4.32                                         3/4"                                                                  0.736                       74.9         4.14                                 0.992                       59.8         3.34                                         1"                                                                    0.992                       59.8         3.34                                 1.45                        46.8         16.88                                        11/2 "                                                                1.44                        50.1         6.79                                 1.95                        40.1         34.4                                         2"                                                                    1.95                        40.1         30.9                                 ______________________________________                                    

The as extruded polycarbonate sheets of thickness 1, 11/2 and 2 incheshave a haze greater than 30%, are translucent not transparent, andcannot be determined by the above procedure. The refractive index of thepolycarbonate sheet of about 1/8 inch thickness was found to be 1.61 andfor all the sheets 1.61-1.62 as determined by the microscopic method ofASTM D542.

Additionally, to determine optical distortion, a grid pattern (1-inchsquares) was photographed through optical flat glass, common windowglass, standard extruded polycarbonate sheet and pressure polishedpolycarbonate sheet using a commercial 14 inch camera, f/6.3 stoppeddown to f/4.5 2 at 1 kw scoop lights, a camera to sample distance of 5feet and a sample to grid distance of 11 feet. The results were that thepressure polished polycarbonate had considerably less distortion thanthe window glass but was not quite as distortion-free as the opticalflat glass.

It should be noted that the polycarbonate sheets used in the tests setforth in the Table hereinabove for all thicknesses of the pressurepolished sheets and for the as extruded (unpolished) sheets up to 1/2inch in thickness were made from Lexan 9030-112, a u.v. stabilizedpolycarbonate resin, a product of General Electric Company. The asextruded (unpolished) sheets above 1/2 inch in thickness were made fromLexan 101-111, a high melt viscosity polycarbonate resin, also a productof General Electric Company.

Thus, it will be seen that the pressure polished polycarbonate hasoptical properties superior to those of the as extruded sheet andoptical distortion approaching that of optical flat glass providingdesirable see-through clarity and lack of distortion for manyapplications. It should be noted that the superior optical propertiesalso result from the fact that the outer surfaces of the sheet arerendered substantially parallel during the pressure polishing thereof.

It is within the purview of this invention to include polycarbonatesheets pressure polished by the aforedescribed process usingpolycarbonate resins containing transparent color tints such as amber,smoke gray, smoke bronze and the like, in which case the opticalproperties thereof will be somewhat reduced in comparison to theuntinted resin sheets. However, the optical properties of luminoustransmittance, distortion and haze of the tinted pressure polishedsheets will in all cases be superior to the corresponding tinted asextruded (unpolished) polycarbonate resin sheets. The differences in theproperties of luminous transmittance and haze will be in the same orderof magnitude as those appearing in the Table hereinbefore.

While preferred embodiments of the invention have here been shown anddescribed, it will be understood that minor variations may be madewithout departing from the spirit of the invention.

What is claimed is:
 1. A sheet of extruded unfilled transparentpolycarbonate having opposed pressure polished mirror finishedsubstantially parallel surfaces which are substantially free of ripples,wiggles, extrusion die lines and stresses and whose percent luminoustransmittance is greater than, its percent haze less than and itsdistortion less than the as extruded sheet of the same thickness, saidpressure polished sheet having a thickness of about 0.040 inch, aluminous transmittance of about 87.5% to 95.4% and a haze of about 1.68%to 2.74%, said sheet being made by first preheating the as extrudedsheet between a pair of mirror polished plates at a pressure of zero toa maximum of 50 psi to the heat distortion temperature of the sheet toexpel air entrapped between the sheet and the plates, then increasingthe pressure to about 300 to 500 psi while allowing the temperature torise to about 310°-330°F to transfer the mirror finish of the plates tothe sheet surfaces while substantially eliminating the ripples, wigglesand extrusion die lines and stresses, and then cooling the sheet whilereducing the pressure to fix the mirror finish in the sheet surfaces asthe sheet contracts and solidifies.
 2. A sheet of extruded unfilledtransparent polycarbonate having opposed pressure polished mirrorfinished substantially parallel surfaces which are substantially free ofripples, wiggles, extrusion die lines and stresses and whose percentluminous transmittance is greater than, its percent haze less than andits distortion less than the as extruded sheet of the same thickness,said pressure polished sheet having a thickness of about 1/8 inch, aluminous transmittance of about 86.5% to 90.0% and a haze of about 1.62%to 2.67%, said sheet being made by first preheating the as extrudedsheet between a pair of mirror polished plates at a pressure of zero toa maximum of 50 psi to the heat distortion temperature of the sheet toexpel air entrapped between the sheet and the plates, then increasingthe pressure to about 300 to 500 psi while allowing the temperature torise to about 310°-330°F to transfer the mirror finish of the plates tothe sheet surfaces while substantially eliminating the ripples, wigglesand extrusion die lines and stresses, and then cooling the sheet whilereducing the pressure to fix the mirror finish in the sheet surfaces asthe sheet contracts and solidifies.
 3. A sheet of extruded unfilledtransparent polycarbonate having opposed pressure polished mirrorfinished substantially parallel surfaces which are substantially free ofripples, wiggles, extrusion die lines and stresses and whose percentluminous transmittance is greater than, its percent haze less than andits distortion less than the as extruded sheet of the same thickness,said pressure polished sheet having a thickness of about 1/4 inch, aluminous transmittance of about 85.1% to 85.7% and a haze of about 2.33%to 2.35%, said sheet being made by first preheating the as extrudedsheet between a pair of mirror polished plates at a pressure of zero toa maximum of 50 psi to the heat distortion temperature of the sheet toexpel air entrapped between the sheet and the plates, then increasingthe pressure to about 300 to 500 psi while allowing the temperature torise to about 310°-330°F to transfer the mirror finish of the plates tothe sheet surfaces while substantially eliminating the ripples, wigglesand extrusion die lines and stresses, and then cooling the sheet whilereducing the pressure to fix the mirror finish in the sheet surfaces asthe sheet contracts and solidifies.
 4. A sheet of extruded unfilledtransparent polycarbonate having opposed pressure polished mirrorfinished substantially parallel surfaces which are substantially free ofripples, wiggles, extrusion die lines and stresses and whose percentluminous transmittance is greater than, its percent haze less than andits distortion less than the as extruded sheet of the same thickness,said pressure polished sheet having a thickness of about 3/8 inch, aluminous transmittance of about 80.7% to 83.0% and a haze of about 2.53%to 2.73%, said sheet being made by first preheating the as extrudedsheet between a pair of mirror polished plates at a pressure of zero toa maximum of 50 psi to the heat distortion temperature of the sheet toexpel air entrapped between the sheet and the plates, then increasingthe pressure to about 300 to 500 psi while allowing the temperature torise to about 310°-330°F to transfer the mirror finish of the plates tothe sheet surfaces while substantially eliminating the ripples, wigglesand extrusion die lines and stresses, and then cooling the sheet whilereducing the pressure to fix the mirror finish in the sheet surfaces asthe sheet contracts and solidifies.
 5. A sheet of extruded unfilledtransparent polycarbonate having opposed pressure polished mirrorfinished substantially parallel surfaces which are substantially free ofripples, wiggles, extrusion die lines and stresses and whose percentluminous transmittance is greater than, its percent haze less than andits distortion less than the as extruded sheet of the same thickness,said pressure polished sheet having a thickness of about 1/2 inch, aluminous transmittance of about 79.0% to 82.0% and a haze of about 2.68%to 3.04%, said sheet being made by first preheating the as extrudedsheet between a pair of mirror polished plates at a pressure of zero toa maximum of 50 psi to the heat distortion temperature of the sheet toexpel air entrapped between the sheet and the plates, then increasingthe pressure to about 300 to 500 psi while allowing the temperature torise to about 310°-330°F to transfer the mirror finish of the plates tothe sheet surfaces while substantially eliminating the ripples, wigglesand extrusion die lines and stresses, and then cooling the sheet whilereducing the pressure to fix the mirror finish in the sheet surfaces asthe sheet contracts and solidifies.
 6. A sheet of extruded unfilledtransparent polycarbonate having opposed pressure polished mirrorfinished substantially parallel surfaces which are substantially free ofripples, wiggles, extrusion die lines and stresses and whose percentluminous transmittance is greater than, its percent haze less than andits distortion less than the as extruded sheet of the same thickness,said pressure polished sheet having a thickness of about 3/4 inch, aluminous transmittance of about 74.0% to 74.9% and a haze of about 4.14%to 4.32%, said sheet being made by first preheating the as extrudedsheet between a pair of mirror polished plates at a pressure of zero toa maximum of 50 psi to the heat distortion temperature of the sheet toexpel air entrapped between the sheet and the plates, then increasingthe pressure to about 300 to 500 psi while allowing the temperature torise to about 310°-330°F to transfer the mirror finish of the plates tothe sheet surfaces while substantially eliminating the ripples, wigglesand extrusion die lines and stresses, and then cooling the sheet whilereducing the pressure to fix the mirror finish in the sheet surfaces asthe sheet contracts and solidifies.
 7. A sheet of extruded unfilledtransparent polycarbonate having opposed pressure polished mirrorfinished substantially parallel surfaces which are substantially free ofripples, wiggles, extrusion die lines and stresses and whose percentluminous transmittance is greater than, its percent haze less than andits distortion less than the as extruded sheet of the same thickness,said pressure polished sheet having a thickness of about 1.0 inch, aluminous transmittance of about 59.8% and a haze of about 3.34%, saidsheet being made by first preheating the as extruded sheet between apair of mirror polished plates at a pressure of zero to a maximum of 50psi to the heat distortion temperature of the sheet to expel airentrapped between the sheet and the plates, then increasing the pressureto about 300 to 500 psi while allowing the temperature to rise to about310°-330°F to transfer the mirror finish of the plates to the sheetsurfaces while substantially eliminating the ripples, wiggles andextrusion die lines and stresses, and then cooling the sheet whilereducing the pressure to fix the mirror finish in the sheet surfaces asthe sheet contracts and solidifies.
 8. A sheet of extruded unfilledtransparent polycarbonate having opposed pressure polished mirrorfinished substantially parallel surfaces which are substantially free ofripples, wiggles, extrusion die lines and stresses and whose percentluminous transmittance is greater than, its percent haze less than andits distortion less than the as extruded sheet of the same thickness,said pressure polished sheet having a thickness of about 11/2 inches, aluminous transmittance of about 46.8% to 50.1% and a haze of about 6.79%to 16.88, said sheet being made by first preheating the as extrudedsheet between a pair of mirror polished plates at a pressure of zero toa maximum of 50 psi to the heat distortion temperature of the sheet toexpel air entrapped between the sheet and the plates, then increasingthe pressure to about 300 to 500 psi while allowing the temperature torise to about 310°-330°F to transfer the mirror finish of the plates tothe sheet surfaces while substantially eliminating the ripples, wigglesand extrusion die lines and stresses, and then cooling the sheet whilereducing the pressure to fix the mirror finish in the sheet surfaces asthe sheet contracts and solidifies.
 9. A sheet of extruded unfilledtransparent polycarbonate having opposed pressure polished mirrorfinished substantially parallel surfaces which are substantially free ofripples, wiggles, extrusion die lines and stresses and whose percentluminous transmittance is greater than, its percent haze less than andits distortion less than the as extruded sheet of the same thickness,said pressure polished sheet having a thickness of about 2.0 inches, aluminous transmittance of about 40.1% and a haze of about 30.9% to34.4%, said sheet being made by first preheating the as extruded sheetbetween a pair of mirror polished plates at a pressure of zero to amaximum of 50 psi to the heat distortion temperature of the sheet toexpel air entrapped between the sheet and the plates, then increasingthe pressure to about 300 to 500 psi while allowing the temperature torise to about 310°-330°F to transfer the mirror finish of the plates tothe sheet surfaces while substantially eliminating the ripples, wigglesand extrusion die lines and stresses, and then cooling the sheet whilereducing the pressure to fix the mirror finish in the sheet surfaces asthe sheet contracts and solidifies.